The present invention relates to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Most computer-to-plate systems, particularly those that rely on laser exposure, do not utilize traditional metal-based plates. Such plates remain the standard for most of the long-run printing industry due to their durability and ease of manufacture, utilizing a heavy, metal base with a textured (e.g., grained and/or anodized) hydrophilic surface. But that surface, while durable in the context of printing, is nonetheless vulnerable to heat damage; if the texture is disrupted or smoothed, the plate will no longer adsorb fountain solution and therefore cannot function lithographically.
One approach toward adapting traditional metal-based plates to laser imaging is disclosed in U.S. Pat. No. 5,493,971. A lithographic printing construction in accordance with this patent includes a grained-metal substrate, a protective layer that can also serve as an adhesion-promoting primer, and an ablatable oleophilic surface layer. In operation, imagewise pulses from an imaging laser (typically emitting in the near-infrared, or xe2x80x9cIRxe2x80x9d spectral region) interact with the surface layer, causing ablation thereof and, probably, inflicting some damage to the underlying protective layer as well. The imaged plate may then be subjected to a solvent that eliminates the exposed protective layer, but which does no damage either to the surface layer or the unexposed protective layer lying thereunder. By using the laser to directly reveal only the protective layer and not the hydrophilic metal layer, the surface structure of the latter is fully preserved; the action of the solvent does no damage to this structure.
A related approach is disclosed in published PCT Application Nos. US99/01321 and US99/01396. A printing member in accordance with this approach has a grained metal substrate, a hydrophilic layer thereover, an ablatable layer, and an oleophilic surface layer. The surface layer is transparent to imaging radiation, which is concentrated in the ablatable layer by virtue of that layer""s intrinsic absorption characteristics and also due to the hydrophilic layer, which provides a thermal barrier that prevents heat loss into the metal substrate. As the plate is imaged, ablation debris is confined beneath the surface layer; and following imaging, those portions of the surface layer overlying imaged regions are readily removed. Because the hydrophilic layer survives the imaging process, it can serve the printing function normally performed by grained aluminum, namely, adsorption of fountain solution.
Both of these approaches involve xe2x80x9cpositive-workingxe2x80x9d or xe2x80x9cpositive-imagingxe2x80x9d constructions in the sense that inherently ink-receptive areas receive laser output and are ultimately removed, revealing the hydrophilic layer that will reject ink during printing; in other words, the xe2x80x9cimage areaxe2x80x9d is selectively removed to reveal the xe2x80x9cbackground.xe2x80x9d Such plates, also referred to as xe2x80x9cindirect-write,xe2x80x9d require substantial amounts of exposure, since in most printing jobs the background area greatly exceeds that of the image. This may tax certain types of imaging systems. Indeed, all of these constructions rely on ablationxe2x80x94complete release of a layer by catastrophic overheatingxe2x80x94as an imaging mechanism, which requires imaging equipment capable of high (and therefore potentially vulnerable) output power levels.
The present invention provides xe2x80x9cnegative-workingxe2x80x9d or xe2x80x9cnegative-imagingxe2x80x9d wet lithographic plates that obviate the need for ablation as an imaging mechanism. These combine the benefits of simple construction, the ability to utilize traditional metal base supports, and amenability to imaging with low-power lasers that need not impart ablation-inducing energy levels. In one embodiment, a lithographic printing member in accordance with the invention comprises a metal support having a hydrophilic surface, and a polymeric layer thereover that absorbs imaging radiation and is soluble in a liquid to which ink will not adhere (e.g., fountain solution). Ordinarily, the polymeric layer is removable by fountain solution. In response to absorbed imaging radiation, however, it becomes permanently bound to the metal support and resists removal, serving as an ink-carrying oleophilic layer during printing. In another embodiment, the entire polymeric layer is removed, but the effect of imaging is to transform the hydrophilic metal surface into a surface that is hydrophobic and oleophilic.
A conventional gum, combined with an absorber of imaging radiation (e.g., in pigment or dye form, or as a chromophore integral within the polymer matrix), may be utilized as the polymeric layer. Indeed, it has been found that the gum arabic traditionally used to protect textured lithographic aluminum from environmental damage may serve as the basis for a suitable polymer formulation. Gum arabic is a protective colloid and emulsifier that protects oxide surfaces against further oxidation.
The oleophilicity of the polymeric layer may be enhanced by addition of a metal such as copper, either as an additive within the polymer matrix or, more desirably, as a very thin layer thereover. In the latter case, the heat produced by imaging of the polymeric layer causes the metal to integrate into the fused matrix, imparting oleophilicity thereto.
Because the plates of the present invention are xe2x80x9cnegative-working,xe2x80x9d only the areas that will ultimately accept ink receive laser output, so stress on the imaging equipment is minimized; and because the exposed layer is at most anchored (rather than being removed), overall power requirements may be less than those of ablation systems.
It should be stressed that, as used herein, the term xe2x80x9cplatexe2x80x9d or xe2x80x9cmemberxe2x80x9d refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution; suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement.
Furthermore, the term xe2x80x9chydrophilicxe2x80x9d is used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water for conventional ink systems, aqueous and non-aqueous dampening liquids, and the non-ink phase of single-fluid ink systems. Thus, a hydrophilic surface in accordance herewith exhibits preferential affinity for any of these materials relative to oil-based materials.